The present invention is directed to a highly efficient water distillation process and an apparatus thereof and more particularly, the present invention is directed to a highly efficient water distillation process used in the thermal recovery of heavy oil which minimizes fouling and scaling of operating equipment over long periods of operation.
Throughout the many regions in the world, heavy oil, a hydrocarbon material having much higher viscosity or lower API gravity (less than 20xc2x0 API, typically 7xc2x0 to 12xc2x0 API) than conventional petroleum crude, is more difficult to recover and requires enhanced thermal stimulation techniques of the subsurface reservoir to produce. More particularly, in areas of Western Canada heavy oil producers use a technique of injecting high pressure steam into the reservoir at typical pressures of about 1,500 to 3,000 psig, and in some cases as low as 150 psig. The steam heat energy is generated by an apparatus known as a steam generator to a 60 to 80% steam quality and injected into vertical or horizontal well arrangements to reduce the heavy oil viscosity. The flowable heavy oil is collected in adjacent producing wells and a combination of heavy oil, oil/water emulsion, condensed steam and formation brackish water (known as produced water) is produced to the surface. Using surface facilities, heavy oil is separated from the production fluids and recovered for commercial sale. The produced water, typically recovered at water/oil ratios of 2 to 5, is currently disposed of in subsurface disposal wells. Makeup water from an authorized ground water source is used to makeup the steam generator feed water demand. Typically the makeup water receives minimum treatment to reduce hardness and silica compounds to avoid scaling of the steam generator heat exchange surfaces and prevent a safety hazard. In some facilities, the concentrated brine water from the steam generator discharge is separated from the reservoir injection steam and disposed of in suitable deep disposal wells. This concentrated brine water can also be referred to as high pressure blowdown. This prevents excess and unnecessary hot water from being injected into the reservoir during the steam stimulation operation. Typical current heavy oil recovery practices using the steam injection technique are referred to as Cyclic Steam Stimulation (CCS or Huff n"" Puff), Steam Assisted Gravity Drained (SAGD) and Steam Assisted Gas Pushed (SAGP) methods.
Public and regulatory pressures require that heavy oil producers implement water recovery and reuse practices and in some facilities a zero effluent discharge is required. This means that 100% of the water used be recovered and reused and the elimination of offsite disposal of effluent streams. The produced water, recovered from the oil separation facility and the HP (high pressure) steam separators, contains hardness components, dissolved and suspended silica and colloidal compounds (clay) and dissolved solids such as sodium chloride. If this brackish water is recycled without treatment, the operation of the steam generators is at risk due to fouling and scaling.
A further problem encountered with the current heavy oil recovery practices using steam injection, is that as the operating temperatures of producing reservoirs are increased from 230xc2x0 F. to greater than 400xc2x0 F. to enhance the heavy oil recovery, the temperature of the recovered production fluids (oil and water) increase. To facilitate the common practice of atmospheric oil and water separation, significant quantities of steam is created when the fluid pressure is reduced. This steam is typically condensed by an external means, such as an air cooler to recover the condensed water. The heat energy of the condensing steam is discharged to the atmosphere and wasted.
Until the advent of the present invention combining the recovery of waste heat energy with a highly efficient and non-scaling water distillation process, the recycle of heavy oil produced water and concentrated brine disposal streams has been technically and commercially restricted.
Generally speaking, water distillation is a highly effective method of vaporizing a pure water distillate and recovering a concentrated liquid or solid containing a large quantity of non-volatile components. This process method can be an effective means to recover clean pure water from contaminated sources. However, water distillation processes typically have several problems not the least of which can be fouling or scaling of the apparatus with minerals or other components from the fluid being distilled. Common scaling compounds consist of calcium, magnesium and silicon. Fouling, or to a greater extent, scaling of the heat transfer surfaces have a detrimental effect on the capacity of the heat transfer components, causing conventional distillation processes to become inoperable.
In the prior art, Tsuruta, in U.S. Pat. No. 4,566,947, issued Jan. 28, 1986, taught a general distillation process, but did not recognize the key factors necessary for the prevention of fouling or the applicability of the process for treating produced water from heavy oil recovery. The most important passage in the Tsuruta reference is at column 7, beginning at line 55, with respect to FIG. 4, which states:
xe2x80x9cThe method which employs a vapor compressor 307 in this manner is advantageous in a case where the feed liquid gives rise to precipitation of solid upon condensation of its volatile component or clogging with pitch-like material which would cause dangerous accidents or troublesome maintenance and service of the compressor. With the above-described arrangement, only the vapor from the evaporator passes the compressor 307, thus preventing the occurrence of such troubles. The interiors of the line 350 and the reboiler 352 can be maintained in a clean state by the use of a suitable washing means. The foregoing method is advantageous especially when the bottom liquid is water, since it is possible to replenish through the line 353 cheap process water which does not require recovery. When the water which is collected at the tower bottom of 306 does not contain substances which foul the inside of the compressor 307, it may be fed to the evaporator though the line 353 to keep the liquid level in the evaporator constant.xe2x80x9d
FIG. 4 of Tsuruta has been reproduced hereinbelow as well as an additional figure, (Revised FIG. 4), which substantially corresponds to FIG. 4 of Tsuruta, which incorporates Applicant""s apparatus to effect its method.
As is evident from a review of FIG. 4 from the Tsuruta reference, if a forced circulation reboiler circuit were added to U.S. ""947 and a specific vapor ratio defined, the bottom liquid water would contain fouling substances and operate without fouling or scaling the heated surfaces.
In the FIG. 4 illustration of Tsuruta, lines 340 and 353 are not connected. There is no connection from bottom 306 to line 353. Section 306 in the tower is defined as the tower bottom containing bottom liquid with a pre determined ammonia concentration. Tsuruta highlights the fact that the method is advantageous especially when the bottom liquid is water.
Tsuruta clearly states, in the passage noted above and to which emphasis has been added, that as long as the water which is collected in the bottom of the tower does not contain substances which foul, the water may be fed to the evaporator. The instant application is unconcerned as to the nature of the feed stream for fouling the evaporator. The water contaminated with contaminants can be fed directly to the evaporator without any fear or fouling or other damage to the heat exchanger. In effect, this is an exact opposite to what Tsuruta teaches. By consideration of the circuit loop in FIG. 4 of Tsuruta, all of the heated surfaces related to the bottoms liquid in the apparatus never come in contact with anything other than water substantially free of fouling contaminants, which water is used as the primary medium for stripping ammonia out of a mixture of ammonia and water. There is a teaching in Tsuruta in column 3, at lines 19 through 23, it is stated that the bottom liquid in the tower 1 is sent to the reboiler through line 34, the heated bottoms liquid through line 35. Further still, line 20 indicates that the bottoms is heated by receipt of the heat of condensation of the compressed vapor. After an extensive review of the disclosure, there does not appear to be any mention of a vapor or vapor liquid ratio.
If one combines the teachings from column 7, outlined above together with those in column 3, the only result is that the apparatus will foul. Tsuruta, by the combination of these teachings presents information which can only lead to the apparatus fouling. By contrast, the technologies herein effectively provides for a system which can take a contaminant loaded feed stream containing water and deliver this to the apparatus without any fear of fouling the exchanger surface.
This is possible in view of the recognition of nucleate boiling and the importance of this physical phenomenon in maintaining a wetted surface in a circuit containing a heat exchanger. As is known, the nucleate boiling regime for a pool of water at atmospheric pressure is a fairly specific area where individual bubbles form. This has been established in the references Principles of Heat Transfer, Third Edition, Frank Kreith; and, and Heat Transfer, Seventh Edition, J. P. Holman.
From the reference Principles of Heat Transfer, page 498 provides a discussion concerning stable film and nucleate boiling. In this passage, FIG. 10-2 is referenced as illustrating nucleate boiling. It is evident that individual bubbles are formed on the wire illustrated in the figure. This phenomenon is also illustrated in FIG. 9-5 on page 520 of the second reference, Heat Transfer. In this reference, the author actually acknowledges on page 519, that there is considerable controversy concerning the mechanism of nucleate boiling. It has been recognized in the instant case the importance of maintaining nucleate boiling. This concept is important to retain a wetted surface on the heat exchanger and this is what facilitates a feed stream containing any fouling contaminants from coming into contact with the heat exchange surface without any risk of fouling. At vapor fractions of greater than 50%, the heat exchanger will effectively become clogged.
The technology set forth herein provides for treatment of a feed stream containing fouling contaminants. The fouling contaminants in the feed stream can come in direct contact with the heat exchanger surface without any fouling. This latter feature is not possible by Tsuruta""s own admissions. This passage has been set forth above. It is the recognition of the aforementioned principles involved that allows this method to achieve desirable results. Tsuruta is simply not applicable to this invention.
Another common problem with typical water distillation processes is that of the high energy input requirements. Without a source of waste heat energy and a means to effectively recover this input energy, the energy required is equivalent to the latent heat of vaporization of water at a given pressure/temperature. Water distillation, under this condition is not commercially viable forwater remediation applications. Heavy oil producing facilities commonly consist of high energy related fluid streams suitable as sources forwaste heat energy recovery.
Several variables must be considered to overcome the problems with conventional distillation methods. The following three equations describe the basic heat transfer relationships within a water distillation system:
In order to have an efficient distillation system, the quantity of heat exchanged and recovered, Q, expressed by the above stated equations, must be maximized, while at the same time obeying the practical limits for the remaining variables and preventing scaling and fouling. For a given fluid and fluid dynamics within a given heat exchange apparatus, the variables, U, Cp and L are relatively non-variable. Therefore, careful consideration must be given to the variables A, Q/A, LMTD, m, and T1 and T2 to overcome the problems associated with distillation of contaminated water.
To fully overcome the problems related to distilling contaminated water from a heavy oil thermal recovery facility and eliminate scaling, other essential factors must be considered beyond the basic equations stated above:
transforming effective sources of waste heat energy;
the rate by which the heat is transferred within the distillation system, known as heat flux or QAxe2x88x921 (Btu hrxe2x88x921 ftxe2x88x922)
the level of contaminates in the concentrate;
the final boiling point of the concentrate relative to the saturation temperature of the vapor stream;
the degree of supersaturation and level of precipitation of the concentrate; and
level of vaporization of the evaporating stream.
Until the advent of the present invention, effectively recovering waste heat energy from a heavy oil facility and maximizing the quantity of heat transferred and recovered with a water distillation process, without the tendency of fouling or scaling, could not be realized over a long term continuous period.
A process has been developed which is both energy efficient and eliminates the problems of scaling previously encountered in the distillation of contaminated water, contaminated with organics, inorganics, metals, inter alia.
The invention further advances the concepts established in the initial application. Former concepts linked two distinct concepts, consisting of distillation or multiple effect water distillation using vapour recompression and waste heat recovery in combination with a unique heat recovery circuit. It has been found by further combining the recovery of a low grade heat energy source from a heavy oil thermal recovery unit together with a uniquely configured forced convection heat recovery and transfer circuit, that very desirable results can be obtained in terms of maximizing heat transfer, eliminating or minimizing compression power requirements and maintaining the desired forced convection circuit non-conductive to scaling exchangers, which is typically encountered by practicing standard distillation methods.
It has now been found that the use of the waste steam energy from the heavy oil recovery unit can be recovered in the heat transfer circuit and this source of low grade energy, most commonly discharged as excess energy or unrecoverable energy, is employed to reduce or eliminate the quantity of requisite compression to treat waste water and significantly reduce the commercial benefits of the process.
By this methodology, a source of waste energy is available in the HP blowdown liquid from the HP steam separator, which is flashed to low pressure to form low grade steam and hot brine water at about 10 to 15 psig. The LP (low pressure) steam is used in the heated separator as the thermal source to evaporate off distilled water, which further condenses to high quality boiler feed water. The hot concentrated blowdown is used to preheat the inlet produced water feed stream prior to entering the heated separator.
Further, a significant source of waste energy is available from the depressurizing of production fluids returning from the heavy oil reservoir. The production fluids returning from the reservoir at, typically 50 to 300 psig, are depressurized to near atmospheric in a degassing separator. The oil/water production fluids are transferred to the conventional atmospheric oil/water separation facility, commonly known to those skilled in the art. Waste energy can be extracted by two methods. If lift gas is not used in the heavy oil recovery operation and only a minimal quantity of associated gas is present in the production fluids after the well head, waste steam is separated off the degassing vessel and delivered to the high efficiency distillation unit for waste heat energy recovery. If lift gas is used in the well bore to assist with the production of the heavy oil, and/or there is a relatively high level of associated gas present in the production fluids, then the waste heat energy can be recovered using any suitable heat exchange means and transferred by way of a heat medium fluid to the high efficiency distillation unit for waste heat recovery. In this example, the cooled production fluids are degassed in the degassing vessel without appreciable steam losses. The current state of the art for thermal stimulation techniques is to drive the reservoir harder to enhance heavy oil recovery, thereby resulting in higher production fluid temperatures at the producing well heads. These temperatures are reaching levels beyond the typical 230xc2x0 F. to 400xc2x0 F. and even 500xc2x0 F. Therefore, significant recoverable waste heat energy is available as a source for the high efficiency water distillation unit.
One object of the present invention is to provide an improved efficient produced water recovery process for distilling water containing organic, inorganic, metals or other contaminant compounds with the result being a purified water fraction devoid of the contaminants which additionally does not involve any scaling of the distillation apparatus.
A further object of one embodiment of the present invention is to provide a method of recovering energy for treating water used in heavy oil recovery in a reservoir containing heavy oil and water comprising in combination, the steps of:
a) providing a water feed stream;
b) treating the water feed stream to generate a steam fraction and a liquid fraction;
c) providing a steam separator for separating the steam fraction and the liquid fraction;
d) separating the steam fraction and the liquid fraction;
e) providing an oil-water separator and a water distillation apparatus;
f) injecting the reservoir with the steam fraction;
g) collecting heavy oil and produced water from the reservoir in the oil-water separator;
h) separating the heavy oil and produced water from the separator;
i) providing thermal energy contained in the liquid fraction to the water distillation apparatus; and
j) treating the produced water with the water distillation apparatus.
An even further object of one embodiment of the present invention is to a method of recovering energy for treating water used in heavy oil recovery in a reservoir containing heavy oil and water, comprising in combination, the steps of:
a) providing a water feed stream;
b) treating the water feed stream to generate a steam fraction and a liquid fraction;
c) providing a steam separator for separating the steam fraction and the liquid fraction;
d) separating the steam fraction and the liquid fraction;
e) providing an oil-water separator and a water distillation apparatus;
f) injecting the reservoir with the steam fraction;
g) depressurizing heavy oil, produced water to form steam exiting from the reservoir;
h) transferring energy contained in the steam to the water distillation apparatus; and
i) separating the heavy oil and produced water.
An even still further object of one embodiment of the present invention is to a method of recovering energy for treating water used in heavy oil recovery in a reservoir containing heavy oil and water, comprising in combination, the steps of:
a) providing a water feed stream;
b) treating the water feed stream to generate a steam fraction and a liquid fraction;
c) providing a steam separator for separating the steam fraction and the liquid fraction;
d) separating the steam fraction and the liquid fraction;
e) providing an oil-water separator and a water distillation apparatus;
f) injecting the reservoir with the steam fraction;
g) recovering heat energy from the heavy oil and produced water exiting the reservoir with heat exchange means;
h) separating the heavy oil and produced water;
i) providing thermal energy from the heat exchange apparatus to the distillation apparatus
j) providing thermal energy contained in the liquid fraction to the water distillation apparatus; and
k) treating the produced water with the water distillation apparatus.
A still further object of one embodiment of the present invention is to provide a method of recovering energy from a heavy oil recovery facility where the heavy oil is contained in a reservoir, the energy for the treatment of water produced from heavy oil recovery, the method comprising, the steps of:
a) providing a source of steam having a steam fraction and a liquid fraction;
b) providing an oil-water separator and a water distillation apparatus;
c) injecting the reservoir with at least one fraction of the steam fraction and the liquid fraction to recover the heavy oil;
d) collecting heavy oil and water from the reservoir in the oil-water separator;
e) separating the heavy oil and produced water from the separator;
f) providing thermal energy contained in the liquid fraction to the water distillation apparatus; and
g. treating the produced water with the water distillation apparatus.
It has been found that by precisely controlling the ratio of circulating mass in a range of less than 300 to near two times that of the vapor fraction exiting the reboiler, several desirable advantages can be realized:
1. The circulating concentrate through the evaporating side of the reboiler will contain a precisely controlled vapor fraction near 1% to 50% of the mass of the circulating concentrate;
2. By precisely controlling this vapor fraction, the temperature rise of the circulating concentrate remains very low (about 1F) and reboiler heat exchange surfaces remain wetted, at a temperature near that of the circulating concentrated fluid. This reduces the risk of fouling of these surfaces;
3. With this controlled low vapor fraction, the concentrated fluid within the exchanger is subjected to a greatly reduced localized concentration factor of less than 1.1, avoiding localized precipitation of scaling compounds on the exchanger surfaces;
4. As the vapor mass is formed toward the exit of the reboiler, the stream velocities within the exchange passages increase significantly promoting good mixing and thus reducing the risk of fouling;
5. By allowing a controlled vapor fraction in the evaporating fluid, significant heat transfer can be realized through the means of latent heat, without scaling and causing a temperature cross within the heat exchanger;
6. Because the temperature rise of the evaporating side of the reboiler is kept very low, the LMTD of the reboiler is maintained, thereby keeping the input energy requirement very low;
7. By adjusting the heat flux, the temperature of the wet surfaces for condensing and evaporating are maintained near that of the saturated steam condition at the evaporating and condensing conditions. The type of boiling experienced will range from primarily forced convection to stable nucleate boiling off the wetted surfaces; and
8. By providing a reboiling means to absorb low grade waste heat energy from a heavy oil recovery facility, the power required for compression is eliminated, provided sufficient high pressure blowdown is available.
A further object of one embodiment of the present invention is to provide a method of recovering energy from heavy oil treatment for treatment of water produced from the heavy oil recovery, comprising the steps of:
a) providing a high pressure blowdown stream;
b) flashing the high pressure blowdown stream to form a low pressure waste energy stream and concentrated blowdown stream;
c) vaporizing at least a portion of the produced water with the low pressure waste energy stream;
d) preheating the produced water with the concentrated blowdown stream;
e) providing a fluid circuit including a heated separator and a reboiler exchanger in communication;
f) providing a vapor circuit including the heated separator, compressor means and the reboiler exchanger in communication;
g) passing preheated produced water into the heated separator;
h) vaporizing the preheated produced water with the low pressure waste energy and a compressed vapor stream in the reboiler exchanger to generate a vapor fraction and concentrate liquid fraction;
i) treating the vapor fraction formed by the low pressure waste energy with an external condenser means;
j) recovering any remaining portion of the vapor fraction by the compressor means;
k) circulating at least a portion of the concentrate liquid fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of 300 to near 2 to result in a vapor fraction of near 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler exchanger; and
l) collecting the condensed vapor fraction and the waste energy stream substantially devoid of contaminants.
As further advantages to this methodology, the input costs are effectively zero. This is due to the fact that if sufficient low grade waste energy can be made available, there is no requirement for a compressor to treat the produced water. Further still, the method protocol facilitates 100% water recovery and results in a zero waste water effluent solution since the contaminants are converted to solid waste.
Broadly, in one possible embodiment, distilled water is evaporated and passed through a mesh pad to remove any entrained droplets, where it is externally condensed. The waste energy stream enters the reboiler where it is condensed to distillate. The heat energy is transferred to the circulating concentrate from the heated separator where, byway of controlling the mass of circulating concentrate to vapor stream, to a range of less than 300 to near 2, less than 50% vapor or more precisely less than 10% vapor, is generated in the circulating concentrate stream. This vapor formed in the circulating concentrate stream absorbs the transferred heat by latent heat of vaporization, while at the same time not allowing the temperature rise on the circulating concentrate to increase greater than about 1xc2x0 F. The clean distillate water, collected from the external condenser and the reboiler exchanger at condensing temperature and pressure, is returned as high quality steam generator feed water. Simultaneously, a portion of the concentrate stream is removed from the heated separator to control the desired concentration of the non-volatile contaminants. This concentrate blowdown stream at the heated separator temperature and pressure is passed through a preheater to impart the remaining sensible heat energy to the produced water feed stream. Additional pre- and post-treatment techniques can be employed as batch or continuous process methods to remove or contain contaminants, prior to, after or during the distillation operation. Methods of pH control or other chemical additions can be used to ionize volatile components or alter solubility conditions in the concentrate to further enhance the subject distillation process. A substantially high level of distilled water can be recovered, typically in excess of 90% of the water feed stream. With the further addition of a crystallization means, 100% water recovery can be achieved.
In terms of the breadth for this process, the same could be easily employed to any heavy oil recovery operation using steam for thermal stimulation, such as conventional steam flood, cyclic steam stimulation (CSS or Huff n"" Puff), steam assisted gravity drain (SAGD) and steam and gas pushed (SAGP). This listing is by no means exhaustive, but rather exemplary.
Having thus described the invention, reference will now be made to the accompanying drawings illustrating the preferred embodiments.